Ctrl + Tab and Ctrl + Shift + Tab are universal: I can't think of a single Windows application with tabs which doesn't implement them. They're also really easy to hit with one hand (especially if you replace Caps Lock with Ctrl, as a lot of Emacs users do).

If you're in need of a XUL-based instant messenger which tries to be as familiar as possible for Firefox users (that includes keyboard shortcuts amond other things) then you could try Instantbird by the way: http://www.instantbird.com

Jesse: I can't speak to the Windows ecosystem generally, just my particular experience with it. Nevertheless, XUL app consistency is not a given; on Mac, Komodo's shortcuts are different from Firefox's and Thunderbird's.

Benedikt: yup, Instantbird is on the list of apps that I mention using regularly in this post. :-)